Behavioral and electrophysical experiments have shown that horizontal and vertical movements are perceived by separate and independent systems in the fly. We have identified the cells in this system at the level of the lobula plate (third optic ganglion) and identified some other members of the systems in the midbrain and thoracic ganglion. We propose to continue the identification of elements in the systems and to ascertain the electrophysiological processing underlying movement perception. Genetic dissection of the systems will be investigated by anatomical studies on mutants of Drosophila.